Download Prolonged Attenuation of the Reinforcing Strength of

Neuropsychopharmacology (2011) 36, 539–547
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Prolonged Attenuation of the Reinforcing Strength of
Cocaine by Chronic d-Amphetamine in Rhesus Monkeys
Paul W Czoty1, Robert W Gould1, Jennifer L Martelle1 and Michael A Nader*,1,2
1
Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, NC, USA; 2Department of Radiology,
Center for the Neurobiology of Addiction Treatments, Wake Forest University School of Medicine, Winston-Salem, NC, USA
Chronic treatment with the indirect dopamine agonist d-amphetamine can reduce cocaine use in clinical trials and, in preclinical studies in
laboratory animals, attenuates daily cocaine self-administration. The present study extended previous results to conditions designed to
reflect a more clinically relevant experience of cocaine exposure and d-amphetamine treatment. Each morning, monkeys pressed a lever
to receive food pellets under a 50-response fixed-ratio schedule of reinforcement. After determining a dose-response curve for cocaine
(0.003–0.56 mg/kg per injection, i.v.) under a progressive-ratio (PR) schedule of reinforcement in the evening, cocaine self-administration
sessions were suspended and d-amphetamine (0.01–0.056 mg/kg/h, i.v.) was administered continuously for at least 24 days, except during
cocaine self-administration sessions, which were conducted using the PR schedule once every 8 days. When a persistent decrease in selfadministration was observed, the cocaine dose-effect curve was redetermined. Cocaine- and food-maintained responding were also
examined after discontinuation of d-amphetamine. Although individual differences in sensitivity were observed, d-amphetamine produced
selective, qualitatively similar decreases in the reinforcing strength of cocaine in all monkeys that persisted at least 4 weeks. Moreover,
cocaine dose-effect curves were shifted downward and/or to the right. For 2 weeks following discontinuation of d-amphetamine
treatment, the reinforcing strength of cocaine varied within and across individuals, however, on the whole no increased sensitivity was
apparent. These data provide further support for the use of agonist medications for cocaine abuse, and extend the conditions under
which such treatment is successful to those that incorporate clinically relevant patterns of cocaine use and drug treatment.
Neuropsychopharmacology (2011) 36, 539–547; doi:10.1038/npp.2010.185; published online 20 October 2010
Keywords: animal model; cocaine; d-amphetamine; drug abuse; medication development; rhesus monkey
INTRODUCTION
Cocaine abuse persists as a major public health problem, for
which no pharmacotherapy has proven to be sufficiently
effective (Vocci and Ling, 2005). The success of methadone
and nicotine replacement therapies in the treatment of
opiate and nicotine addiction, respectively, has encouraged
efforts to develop an indirect dopamine agonist medication
to treat stimulant abuse (Grabowski et al., 2004a; Rothman
and Glowa, 1995; Herin et al., 2010). Although efforts are
ongoing to develop novel dopamine indirect agonists for
this purpose (eg, Carroll et al., 1999; Platt et al., 2002; Lile
and Nader, 2003; Rothman et al., 2005; Negus et al., 2007),
several clinical studies have supported the safety and
efficacy of d-amphetamine as a treatment for cocaine
*Correspondence: Dr MA Nader, Department of Physiology and
Pharmacology, Wake Forest University School of Medicine, Medical
Center Boulevard, 546 NRC, Winston-Salem, NC 27157-1083, USA,
Tel: + 336-713-7172, Fax: + 336-713-7180,
E-mail: [email protected]
Received 14 July 2010; revised 31 August 2010; accepted 14
September 2010
dependence (eg, Fleming and Roberts, 1994; White, 2000),
including three double-blind, placebo-controlled studies
that used a sustained-release preparation (Grabowski et al.,
2001, 2004b; Shearer et al., 2003).
Chronic treatment with d-amphetamine has also been
found to decrease cocaine self-administration in laboratory
animals under several conditions, including progressiveratio (PR) and second-order schedules of reinforcement, as
well as food-cocaine choice procedures (Negus, 2003; Negus
and Mello, 2003a, b; Chiodo et al., 2008; Chiodo and
Roberts, 2009; Czoty et al., 2010). Importantly, d-amphetamine was administered chronically in these studies. Using
chronic drug administration in preclinical models is critical
not only because chronic administration better reflects
ultimate clinical use, but also because acute administration
of d-amphetamine can enhance the reinforcing effects of
cocaine in self-administration and reinstatement paradigms
(Gerber and Stretch, 1975; de Wit and Stewart, 1983; Barrett
et al., 2004; DC Roberts, personal communication).
Under other conditions in which acute d-amphetamine
was reported to decrease cocaine self-administration,
comparable reductions in food-maintained responding were
d-Amphetamine decreases cocaine self-administration
PW Czoty et al
540
observed (Foltin and Evans, 1999; Mansbach and Balster,
1993). The latter findings highlight the importance of
concurrently studying behavior reinforced by a non-drug
stimulus, disruptions of which may indicate potential for
side effects that decrease compliance.
The present study was designed to further characterize
the effects of chronic d-amphetamine treatment on cocaine
self-administration under a PR schedule of reinforcement in
rhesus monkeys, using a regimen of access to cocaine and
an approach to d-amphetamine treatment designed to
better reflect the clinical experience in several ways. First,
unlike other studies, access to cocaine was suspended
during d-amphetamine administration to model a treatment
scenario in which an addict is able to refrain from using
cocaine during a brief, initial period due to incarceration,
hospitalization, or temporarily enhanced motivation to
quit. Rather than assessing cocaine self-administration
daily, the reinforcing strength of cocaine was assessed
only once every 8 days, permitting 7 consecutive days of
d-amphetamine treatment in the absence of cocaine.
Second, food-reinforced responding was monitored daily
to exclude the possibility that any observed decreases in
cocaine-reinforced responding were due to behavioral
disruption that could indicate a likelihood of side effects
in a clinical population. Third, rather then employing a
group design in which all monkeys received the same
d-amphetamine doses for predetermined lengths of time,
the d-amphetamine dose was adjusted for each subject
based on the observed effect (or lack thereof) on cocainereinforced responding. If no decrease in cocaine selfadministration was observed within 4 weeks, or if tolerance
had developed to an initial decrease, the d-amphetamine
dose was increased until decreases in self-administration
were observed. This approach was implemented to reflect
the clinical reality that individual differences in patients’
sensitivity to medications typically result in adjustments in
dose. When a d-amphetamine dose was reached that
resulted in a prolonged attenuation of the reinforcing
strength of cocaine, other cocaine doses were tested to
generate a dose-effect curve, which was compared with the
curve generated in that monkey before d-amphetamine
treatment. Additional features of the present studies that
more closely recapitulate the clinical situation than most
self-administration procedures include the use of cocaineexperienced (vs drug-naı̈ve) subjects, the use of the PR
schedule to measure the reinforcing strength of cocaine
(vs fixed-ratio, fixed-interval or second-order schedules of
reinforcement that assess presence/absence of reinforcing
effects) and the continued assessment of cocaine selfadministration after termination of d-amphetamine treatment to assess whether discontinuation resulted in specific
effects on behavior such as rebound increases in the
reinforcing effects of cocaine or disruption of foodreinforced responding.
MATERIALS AND METHODS
Subjects and Apparatus
Subjects were four adult male rhesus monkeys (Macaca
mulatta), each prepared with a chronic indwelling
venous catheter and subcutaneous vascular access port
Neuropsychopharmacology
(Access Technologies, Skokie, IL) under sterile surgical
conditions as described previously (Czoty et al., 2006). Two
subjects (R-1429 and R-1425) had B6 months of experience
self-administering cocaine at the outset of the present study
and two subjects (R-1427 and R-1268) had self-administered
cocaine for over 4.5 years. Monkeys were housed individually in sound-attenuating chambers (0.91 0.91 0.91 m;
Plas Labs, Lansing, MI). The front wall of each cubicle
was constructed of Plexiglas to allow the monkey visual
access to the laboratory. Each cubicle was equipped with
two response levers (BRS/LVE, Beltsville, MD). Four
stimulus lights, alternating white and red, were located in
a horizontal row above each lever. A receptacle located
between the levers was connected via tygon tubing to a
pellet dispenser located outside the chamber for responsecontingent delivery of food pellets. Each animal was
fitted with a stainless-steel restraint harness and spring
arm (Restorations Unlimited, Chicago, IL) that attached to
the rear of the cubicle. A peristaltic infusion pump
(Cole-Parmer Instrument Co., Vernon Hills, IL) was located
on the top of the chamber for delivering injections at a rate
of B1.5 ml/10 s. Monkeys received fresh fruit, peanuts, and
vegetables several days per week and water was available ad
libitum. Animal housing and handling and all experimental
procedures were performed in accordance with the 2003
National Research Council Guidelines for the Care and Use
of Mammals in Neuroscience and Behavioral Research and
were approved by the Animal Care and Use Committee of
Wake Forest University. Environmental enrichment was
provided as outlined in the Animal Care and Use
Committee of Wake Forest University Non-Human Primate
Environmental Enrichment Plan.
Food-Reinforced Responding
Monkeys were trained under a reinforcement schedule in
which 50 responses on the left lever resulted in delivery of
one food pellet, ie, a 50-response fixed-ratio (FR 50)
schedule. Under this schedule, white stimulus lights above
the left lever were illuminated and 50 responses resulted in
food pellet delivery, extinguishing of white lights and
illumination of red stimulus lights for 10 s, followed by a
10-sec timeout (TO) period during which no lights were
illuminated and responding had no scheduled consequences. Sessions began at 0830 hours each day and lasted
up to 23 h or until the maximum number of allowed food
reinforcers was earned. Thus, only food was available from
0830 hours to 1500 hours when the self-administration
session began (see below). At that point, food and cocaine
were concurrently available if the monkey had not yet
received the maximum number of pellets. The maximum
number of pellets that could be earned was determined for
each monkey as that required to provide enough food to
maintain body weight, measured at least monthly, at B95%
of free-feeding levels. When monkeys earned fewer than the
maximum number of food pellets, supplementary food
(Purina Monkey Chow) was given at B0800 hours in an
amount calculated to raise the total grams of food to the
desired level. Target food amounts for the monkeys in
the present study were 130 or 140 g for three monkeys and
180–190 g for the fourth.
d-Amphetamine decreases cocaine self-administration
PW Czoty et al
541
Cocaine Self-Administration
Monkeys self-administered ()cocaine HCl (National
Institute on Drug Abuse, Bethesda, MD, dissolved in sterile
0.9% saline) under a PR schedule of reinforcement in
sessions that began at 1500 hours each day. Under
this schedule, white stimulus lights were illuminated above
the right lever and 50 responses on that lever resulted in the
first injection of the maintenance dose of cocaine (0.03 or
0.1 mg/kg per injection in B1.5 ml over 10 s), extinguishing
of white lights and illumination of red stimulus lights for
10 s, followed by a 10-min TO. The response requirement for subsequent injections was determined by the
equation used by Richardson and Roberts (1996):
ratio ¼ [5 e(R 0.2)]5, where e is the mathematical
constant and R is equal to the reinforcer number. For
the present studies, the first response requirement (50
responses) corresponds to the 12th value given by this
equation and was followed by 62, 77, 95, 117, 144, 177, 218,
267, 328, 402, 492, 602, 737, 901, 1102, etc. Sessions ended
when 2 h elapsed without an injection.
agitation, stereotopies, or other unconditioned behavioral
effects.
This procedure (availability of the maintenance dose of
cocaine for one session) was repeated every 8 days until day
24. If, at that time, the number of cocaine injections was
decreased from baseline by B30% or more, treatment was
continued another 8 days. Whether or not treatment had
been extended to 32 days, if no decrease in cocaine selfadministration had been observed, or if tolerance to initial
decreases had developed, the dose of d-amphetamine was
then increased to 0.03 mg/kg/h and the effects of this dose
of d-amphetamine were similarly assessed for 32 days.
If treatment with this dose had no effect or if tolerance
developed, the dose was increased to 0.056 mg/kg/h. When a
dose was reached at which the number of cocaine injections
remained decreased at day 32, higher cocaine doses (up to
0.56 mg/kg per injection) were made available for a single
day at 3-day intervals while d-amphetamine treatment was
continued. At 2 days after the curve had been completed,
d-amphetamine treatment was discontinued and selfadministration of the maintenance dose of cocaine was
again examined on post-treatment days 3, 7, and 15.
Chronic d-Amphetamine Treatment
Initially, 0.03 or 0.1 mg/kg cocaine was made available in
evening PR sessions until responding stabilized (3 consecutive days on which the number of injections was
within 2 of the 3-day mean, with no upward or downward
trend). Subsequently, other doses of cocaine were substituted for the maintenance dose for at least 4 days and
until the number of injections stabilized. The maintenance
dose of cocaine was situated on the ascending limb of the
dose-response curve, permitting the detection of either
increases or decreases in the reinforcing strength of cocaine.
At B0830 hours on the next day, the external part of the
catheter was connected to a syringe in an infusion pump
(Cole-Parmer Instrument, Vernon Hills, IL) outside the
chamber and d-amphetamine was infused at a rate of 0.4 ml/h
such that monkeys received a dose of 0.01 mg/kg/h. Foodreinforced responding was studied throughout treatment.
On day 8, the solution being infused was changed from damphetamine to saline at approximately noon. This time
was selected because it takes B3 h to infuse the volume of
d-amphetamine in the catheter (B1.2 ml). Thus, d-amphetamine continued to be infused until B1455 hours, at which
time the catheter was filled with the maintenance dose
of cocaine. This procedure minimized the amount of
d-amphetamine infused as a bolus when the catheter was
filled with cocaine immediately before the start of the selfadministration session. At 1500 hours, the maintenance
dose of cocaine was again made available for self-administration under the PR schedule of reinforcement, signaled
by the illumination of the white stimulus lights above the
right lever. Thus, d-amphetamine treatment continued
until the start of the cocaine self-administration session.
At B0830 hours on the next day, the catheter was flushed
with heparin/saline solution, after which administration of
the same dose of d-amphetamine was continued. For the
first monkey tested (R-1427) a 6-day interval was used
during treatment with 0.01 mg/kg/h d-amphetamine. During
d-amphetamine treatment, monkeys were observed daily to
assess unconditioned effects such as locomotor activation,
Data Analysis
The dependent variable of primary interest was the number
of cocaine injections earned under the PR schedule of
reinforcement. In addition, the number of food reinforcers
received was recorded in hourly bins. Because individual
differences in sensitivity to d-amphetamine resulted in
different regimens of d-amphetamine treatment in the four
monkeys, individual data are shown.
RESULTS
Baseline Food- and Cocaine-Reinforced Responding
Under baseline conditions, monkeys earned all available
food pellets, typically within the first 3 h of availability (data
not shown); in all cases, all food reinforcers had been
received before cocaine availability. In cocaine self-administration sessions, the number of injections received
increased significantly as a function of the available cocaine
dose, up to 0.56 mg/kg cocaine, in all monkeys (Figure 1,
closed symbols). In all cases, the cocaine self-administration
session had concluded by 0730 hours on the morning
following its 1500 hours start. Typically, when a new cocaine
dose was substituted for the training dose, responding
stabilized within 10 sessions.
Effects of d-Amphetamine on Food- and CocaineReinforced Responding
At no time during the present studies were any overt
unconditioned effects of d-amphetamine noted. These data
are consistent with an earlier study (Czoty et al., 2010), in
which only mild agitation was observed in three of four
subjects during treatment with 0.1 mg/kg d-amphetamine.
The only exception was subject R-1268, which showed
transient motor effects (intermittent periods of laying down
with eyes closed or leaning against cage wall) during days 3
and 4 of 0.056 mg/kg d-amphetamine treatment, which
Neuropsychopharmacology
d-Amphetamine decreases cocaine self-administration
PW Czoty et al
542
Figure 1 Dose-effect curves for cocaine self-administration before (closed symbols) and during (open symbols) d-amphetamine treatment in four
monkeys. Each point is the mean of the last three days of availability of a cocaine dose or saline. When a cocaine dose or saline was made available on more
than one occasion, error bars indicate the SEM. Ordinates: number of cocaine injections earned; abscissae, available cocaine dose.
coincided with disruptions in food-maintained responding
(see below and Figure 2, third panel) and dissipated by day 5.
In subject R-1427, food-reinforced responding was not
affected during 24 days of treatment with 0.01 mg/kg/h
d-amphetamine (Figure 2, top panel). The number of
0.03 mg/kg cocaine injections earned was decreased from
baseline on day 6, but by day 24 had returned to a level
slightly above baseline. When the d-amphetamine dose was
increased to 0.03 mg/kg/h, the number of food reinforcers
earned under the FR 50 schedule decreased to near zero
by day 5; tolerance gradually developed to this effect such
that the number of food reinforcers earned returned to
baseline by day 12. When food-maintained responding
was disrupted, monkeys ate all supplemental chows on the
following day as described previously (Czoty et al., 2010).
When first examined on day 8 of treatment with 0.03 mg/kg/h
d-amphetamine, the reinforcing strength of 0.03 mg/kg
cocaine was decreased substantially. In contrast to the
tolerance that developed to d-amphetamine-induced
decreases in food-reinforced responding, the decrease in
cocaine self-administration persisted for over 4 weeks of
treatment.
Neuropsychopharmacology
Effects of d-amphetamine on the reinforcing strength
of cocaine were qualitatively similar in R-1429 (Figure 2,
second panel). Treatment with the lowest dose of
d-amphetamine for 24 days decreased the number of
0.03 mg/kg cocaine injections earned by B40%. Extending
treatment for an additional 8 days resulted in neither
tolerance to nor augmentation of this effect. Next, the damphetamine dose was increased, and following 2 weeks of
treatment, the number of reinforcers earned on the PR
schedule was reduced to two (data not shown). Shortly
thereafter, however, the catheter lost patency and was
replaced. Because we could not be certain that the decrease
was due to effects of d-amphetamine rather than catheter
failure, 0.03 mg/kg cocaine was again made available until
responding again stabilized (BL2). At this point, treatment
with 0.03 mg/kg/h d-amphetamine was begun again. The
reinforcing strength of cocaine gradually decreased up to day
32. In this monkey, unlike the other subjects, food-reinforced
responding was unaffected by d-amphetamine treatment.
As in R-1427 and R-1429, treatment with 0.01 mg/kg/h
d-amphetamine for 24 days had no effects on foodmaintained behavior and only slightly decreased the
d-Amphetamine decreases cocaine self-administration
PW Czoty et al
543
Figure 2 Cocaine self-administration and food-maintained responding during d-amphetamine treatment in four monkeys. Left ordinates: number of
cocaine injections received; right ordinates: number of food reinforcers earned; abscissae: day of exposure to each condition, which is indicated across the
top of each panel. BL represents average (±SEM) number of injections earned at baseline before d-amphetamine treatment. BL2 for R-1429 indicates a
redetermined baseline after catheter replacement (see Results). The dashed line in each graph indicates the mean number of injections received under
baseline conditions (ie, before d-amphetamine treatment).
Neuropsychopharmacology
d-Amphetamine decreases cocaine self-administration
PW Czoty et al
544
reinforcing strength of cocaine (0.1 mg/kg) in R-1268
(Figure 2, third panel). As observed in R-1427, increasing
the d-amphetamine dose to 0.03 mg/kg/h resulted in
immediate decreases in food- and cocaine-reinforced
responding, and tolerance developed to the effect on foodmaintained responding. Unlike in R-1427, however,
tolerance also developed to the decrease in cocaine selfadministration such that the number of 0.1 mg/kg cocaine
injections earned approached baseline levels by day 32.
When the d-amphetamine dose was further increased to
0.056 mg/kg/h, a transient decrease in food-maintained
responding was again observed, but the initial decrease
in cocaine self-administration was maintained for at least
32 days.
In subject R-1425, treatment with d-amphetamine also
resulted in prolonged decreases in self-administration of
0.03 mg/kg cocaine and a decrease in food-reinforced
responding to which tolerance developed (Figure 2, bottom
panel). In this monkey, however, effects were observed
during treatment with the lowest dose, 0.01 mg/kg/h
d-amphetamine.
Redetermination of Dose-Effect Curves
Once a dose of d-amphetamine was found that decreased
cocaine self-administration for at least 32 days, additional
cocaine doses were made available for a single selfadministration session until a complete dose-effect curve
was determined. In all monkeys, the reinforcing strength of
several cocaine doses was found to be decreased compared
with baseline values, such that the dose-effect curves were
shifted downward and/or to the right (Figure 1, open
symbols).
Recovery of Cocaine Self-Administration After
d-Amphetamine Treatment
Food- and cocaine-maintained responding were examined
in the 2 weeks after termination of d-amphetamine
treatment to characterize the time course of recovery of
cocaine self-administration and to assess whether discontinuation of d-amphetamine treatment resulted in any
rebound increases in the reinforcing strength of cocaine
or disruption of food-maintained responding. Although
individual differences were observed across monkeys
(Figure 3), the reinforcing strength of the maintenance
dose of cocaine generally remained decreased below baseline for the first 3–7 days following discontinuation of
d-amphetamine treatment and was near baseline by day
15 after treatment termination in three of four monkeys; no
disruption of food-reinforced responding was observed in
these subjects. In the fourth monkey (R-1425), the
reinforcing strength of cocaine remained substantially
decreased on day 15 after discontinuing d-amphetamine
treatment, and an initial disruption of food-maintained
responding was observed, which dissipated gradually. It is
noteworthy that the subject which was most sensitive to
d-amphetamine during treatment (R-1425) also showed
prolonged disruptions in food- and cocaine-maintained
responding after termination of treatment.
DISCUSSION
Accumulating evidence from preclinical studies and clinical
trials indicates that d-amphetamine possesses therapeutic
potential in the treatment of cocaine dependence (see
Introduction). The results of the present study extend these
findings to a novel animal model designed to incorporate
features likely to be encountered clinically. In all monkeys, a
dose of d-amphetamine was reached that decreased the
reinforcing strength of cocaine for over 1 month. This effect
was selective for cocaine-maintained responding in that
decreases in food-maintained responding either dissipated
over several days or were absent. When a range of cocaine
doses was examined during d-amphetamine treatment,
dose-effect curves for cocaine self-administration were
shifted downward and/or to the right in all monkeys.
Finally, discontinuation of d-amphetamine treatment was
not associated with disruptions in food-maintained responding in three of four monkeys, and although cocaine
self-administration gradually returned to baseline levels, no
rebound increase in the reinforcing strength of cocaine was
observed.
A primary goal of the present studies was to develop an
animal model of cocaine addiction that incorporated
features of human cocaine use and treatment approaches
that have not been used in current models. The impetus for
this effort was the observation that, although intravenous
self-administration techniques have demonstrated impressive predictive validity with respect to the abuse liability of
drugs (eg, Griffiths et al., 1980; Ator and Griffiths, 2003),
Figure 3 Cocaine- and food-maintained responding after discontinuation of d-amphetamine treatment in individual monkeys. Ordinates, symbols and
lines as in Figure 2; abscissae: days after discontinuation of d-amphetamine treatment.
Neuropsychopharmacology
d-Amphetamine decreases cocaine self-administration
PW Czoty et al
545
techniques designed for this purpose have been less
successful in identifying drugs that will serve as effective
medications for cocaine dependence. Moreover, features
known or recommended to increase predictive validity,
such as chronic treatment with putative pharmacotherapies,
assessment of selectivity of medication effects through
concurrent assessment of non-drug maintained behaviors,
single-subject designs and examination of treatment drug
effects on a range of self-administered cocaine doses
(c.f. Mello and Negus, 1996; Ator and Griffiths, 2003; Haney
and Spealman, 2008), are typically not included in
preclinical assessments. For example, the present studies
used cocaine-experienced monkeys, which would be expected to better model a long-term cocaine user than drugnaı̈ve animals. In addition, unlike the majority of preclinical
studies in which animals self-administer cocaine daily
under limited-access conditions (eg, a predetermined
maximum cocaine intake or amount of time), the present
study examined cocaine self-administration using a PR
schedule, in which the end of the session was determined by
the monkey. Moreover, in the present study cocaine selfadministration was suspended during the first week of
treatment. This feature was implemented to model the
frequent (though not universal) clinical situation, in which a
cocaine addict is able to abstain from using cocaine for
a brief period of time during the initial phase of treatment
due to hospitalization, incarceration, or motivation to
remain abstinent when starting treatment. It is unlikely
that the decreases in cocaine self-administration were
simply due to discontinuation of cocaine alone or sporadic
behavioral testing. In previous studies using identical
procedures, discontinuation of self-administration sessions
for up to 1 week did not alter the cocaine dose-response
curve (Czoty et al., 2006). An additional advantage, from a
pharmacological point of view, is that suspending cocaine
treatment provides a more pure assessment of the effects of
the pretreatment drug, as neuroadaptations that are
ostensibly induced by chronic d-amphetamine treatment
are not affected by cocaine during this initial period.
In addition to providing a more clinically relevant pattern
of access to cocaine, the present studies employed an
approach to drug treatment designed to better reflect
clinical use. The response of each subject to d-amphetamine
treatment was monitored individually and adjustments to
the treatment regimen (ie, increasing the d-amphetamine
dose) were made based on the individual’s response (ie, a
single-subject design, c.f. Ator and Griffiths, 2003). This
approach contrasts with the vast majority of animal selfadministration studies, which use a group design in which
all subjects receive a predetermined range of doses and data
are averaged across animals for analysis. In the present
study, although individual differences in sensitivity to
d-amphetamine necessitated that different doses were
effective, d-amphetamine produced similar effects in all
monkeys. Had data for each d-amphetamine dose been
averaged across subjects, this conclusion would have been
obscured. More importantly, an individualized approach to
treatment, with adjustments in medication dose guided by
efficacy and side effects, is more similar to a physician’s
approach to treating a patient.
Perhaps most critically, the present studies examined
chronic administration of d-amphetamine, unlike many
preclinical studies that assess the effects of acute drug
pretreatments on cocaine self-administration. In addition to
the face validity provided by using chronic treatment, it has
become apparent that d-amphetamine has opposite effects
on cocaine administration when administered acutely vs
chronically. Whereas chronic regiments of d-amphetamine
have been consistently shown to decrease cocaine selfadministration across a range of conditions, acute administration of d-amphetamine can enhance the reinforcing
effects of cocaine in self-administration and reinstatement
paradigms (eg, de Wit and Stewart 1983; Barrett et al.,
2004). Contrasting effects of acute vs chronic drug
treatment on cocaine self-administration have also been
observed with other dopaminergic drugs, including the
dopamine D1 receptor antagonist ecopipam (eg, Romach
et al., 1999; Haney et al., 2001) and the low-efficacy D2
receptor agonist aripiprazole (Sorensen et al., 2008; Bergman, 2008; Thomsen et al., 2008).
Chronic treatment also proved critical to assessing the
selectivity of d-amphetamine’s effects on drug- vs foodmaintained behavior, which was assessed by comparing
self-administration to monkeys’ daily responding maintained by presentation of food pellets on a separate
response lever. In three of four monkeys, treatment with
the dose of d-amphetamine that proved effective in
decreasing the reinforcing strength of cocaine produced
decreases in food-maintained responding. However, in all
cases tolerance developed to this effect while cocaine selfadministration remained decreased, similar to the findings
of Negus and Mello (2003b). As observed in a previous
study that examined 5 days of d-amphetamine treatment
(Czoty et al., 2010) effects on food-maintained responding
were likely due to disruptive effects of d-amphetamine on
responding rather than to a reduction in appetite or the
appetitive value of food pellets, as monkeys routinely took
and ate chows and preferred foods when offered by a
technician. Taken together, these data suggest that damphetamine treatment may be associated with some side
effects during initial treatment, but that tolerance is likely to
develop to these effects, whereas the ability of d-amphetamine to reduce the reinforcing strength of cocaine would be
maintained. One caveat to this conclusion is that food and
cocaine were available under different schedules of reinforcement, which may have contributed to the apparent
selectivity of d-amphetamine. However, Negus (2003)
studied cocaine-food choice in which reinforcers were
available under different FR schedules and found that
d-amphetamine selectively reduced cocaine choice. Finally,
in the present study, d-amphetamine was administered
continuously to provide a near-constant level of drug in the
brain. Unlike the study by Negus (2003), d-amphetamine
treatment was terminated before cocaine availability.
Considering the elimination half-life is B4.5 h for intravenous d-amphetamine (Beckett and Rowland, 1965), appreciable levels of drug were most likely present during most of
the session. It would be possible and worthwhile in future
studies to examine whether comparable effects would be
observed with different treatment regimens, including those
that more accurately reflect the pharmacokinetics of drugs
taken once daily by humans.
Once a d-amphetamine dose was identified that decreased
the reinforcing strength of the maintenance dose of cocaine,
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d-Amphetamine decreases cocaine self-administration
PW Czoty et al
546
self-administration of higher cocaine doses was assessed
while d-amphetamine treatment was continued. This phase
of the experiment served two purposes. First, studying a
single cocaine dose generates ambiguous data with respect
to the interaction between d-amphetamine and cocaine; a
decrease in self-administration could be interpreted as
resulting from sedation, toxicity, or behavioral disruption
as well as a decrease in the reinforcing effects of cocaine.
Thus, a range of cocaine doses was studied in combination
with d-amphetamine to gain a more complete understanding of the drug interaction. Second, it was deemed
important to determine whether or not the effects of damphetamine were insurmountable. An important clinical
consideration
regarding
use
of
a
medication
that decreases cocaine use is whether that effect could
be overcome by taking higher cocaine doses. In addition to
circumventing the therapeutic effects of the treatment drug,
increasing the ingested cocaine dose in combination with an
agonist therapy could result in lethal additive effects
including cardiovascular crises or seizures. In the present
studies, however, the ability of d-amphetamine to attenuate
the reinforcing strength of cocaine could not be fully
surmounted by increasing the cocaine dose. That is, in the
presence of d-amphetamine, no dose of cocaine resulted in
a number of injections as high as the dose that had
peak reinforcing strength under baseline conditions. In all
monkeys, chronic d-amphetamine treatment resulted in a
downward and/or rightward displacement of the cocaine
dose-effect curve compared with a dose-effect curve
generated before d-amphetamine treatment. A caveat to
this description of effects is that higher cocaine doses, in
combination with d-amphetamine, may have resulted in a
number of cocaine injections comparable to that seen at
baseline. However, in previous studies in rhesus monkeys
using this PR schedule (eg, Lile et al., 2003; Martelle et al.,
2008; Czoty et al., 2010), dose-effect curves have been
observed to plateau or have a descending limb when doses
X0.56 mg/kg were tested (as seen in Figure 1 for R-1268 and
R-1425). Another reason doses 40.56 mg/kg were not
examined was the concerns regarding toxicity when made
available in combination with d-amphetamine. Finally,
when d-amphetamine treatment was discontinued, cocaine
self-administration gradually returned to baseline levels as
observed previously (Negus and Mello, 2003b; Czoty et al.,
2010). Importantly, no rebound increases in cocaine selfadministration were observed after d-amphetamine treatment, which could be hypothesized under a PR schedule of
reinforcement if termination of d-amphetamine treatment
resulted in a withdrawal state in which monkeys were
hypersensitive to cocaine. Moreover, no disruptions in
food-maintained responding were observed in three of four
monkeys, providing preliminary evidence that termination
of treatment with amphetamine-like drugs may not lead to a
discontinuation syndrome, sometimes observed with antidepressant drugs that act on brain monoamine systems (eg,
Haddad, 1998).
Taken together, the present results extend the conditions
under which d-amphetamine selectively decreases cocaine
self-administration. Importantly, several modifications to
typical self-administration studies were employed in this
model that render the experimental conditions more like
the clinical situation, although there are many other aspects
Neuropsychopharmacology
that have not been, and possibly cannot be, incorporated
into animal models (eg, uniquely human negative consequences of drug use on employment and progressive
psychiatric consequences). With regard to animal models,
increasing face validity does not guarantee enhanced
predictive validity. However, refining the model so that
key features are more similar to those experienced by
human cocaine users may strengthen predictive validity by
increasing the overlap with neurobiological substrates and
mechanisms involved in human drug use and dependence.
Overall, we hypothesize that, by more closely resembling
key clinical factors of cocaine use and treatment, the model
described here will be better able to identify drugs that will
serve as effective medications for cocaine dependence
compared with typical animal models. Testing this hypothesis will require assessment of a variety of drugs that have
and have not demonstrated success in the clinic and/or
in typically used laboratory animal self-administration
procedures. The current results with d-amphetamine are
an encouraging sign that data obtained from this model will
be concordant with clinical studies.
ACKNOWLEDGEMENTS
We thank Nicholas Garrett for his assistance in completing
these studies. This research was supported by NIDA Grant
P50 DA 06634.
DISCLOSURE
The authors declare no conflict of interest.
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